Scavenger Resins

The concept of selective sequestration of nonproduct species has been first proved using solid-supported scavengers with electrophilic and nucleophilic character in amine acylation, amine alkylation and reductive amination protocols [32]. In the meantime, a wide range of scavenger reagents are available commercially from various suppliers. The structures and functions of these scavenger resins are shown in Table 1.

Recently, a number of scavenger resin approaches have appeared in the literature. For the synthesis of 4000 ureas (400 pools of 10-compound mixtures) [33], a solid-supported amino nucleophile was used to quench the excess of isocyanates, yielding the desired products in good purity. A similar concept was employed in the synthesis of 2-thioxo-4-dihydropyrimidi-nones [34] using aminomethylated polystyrene beads to quench isothiocyanates as well as aldehydes. For quenching an excess of amine in the synthesis of 2,6,9- trisubstituted purines [35], formyl polystyrene beads were used to form the corresponding polymer-bound imine, which was filtered off. Furthermore, a pyrazole synthesis with polymer-supported quench (PSQ) purification was described [36]. Primary amine 47, isocyanate 48 and tertiary amine 49 supported on a polymer were used to quench an excess of acids, and an excess of hydrazine, as well as to trap HC1 and acid impurities respectively. The synthesis, as well as the purification steps, are shown in Scheme 19.

Scheme 19

A very similar polymer-supported quench methodology has been used for the synthesis of dihydropyridones and derivatives of dihydropyridones [37], Furthermore, the development of a new scavenger resin for amines has been published recently [38]. The author attached isatoic anhydride 53 to Merryfield resin by alkylation on its nitrogen, and used this resin 54 to remove excess of primary and secondary aliphatic amines (Scheme 20).

Scheme 20

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